1. Field of the Invention
The present invention relates to a high-pressure discharge lamp incorporating a plurality of arc tubes in the outer envelope and the method of operating.
2. Description of the Related Art
During the operation of a high-pressure metal vapor discharge lamp like a high-pressure sodium lamp or a metal halide lamp, for example, normally the internal pressure of the arc tube rises beyond 1 atmospheric pressure. As a result, after once turning the above mentioned discharge lamp off, in order to again light up the discharge lamp, the controller needs to wait a certain period of time to initiate discharge until the arc tube is cooled off to some extent allowing the mercury and luminous metal to condensate and decrease pressure in the arc tube. For example, in order to re-start a high-pressure sodium lamp with an external ignitor, normally it takes about one minute. On the other hand, in order to reactivate a metal halide lamp, normally it takes more than 10 minutes, and yet, even after being reactivated, at least several minutes are required until the luminous output is fully stabilized.
As a result, when power service is momentarily interrupted, unlike an incandescent lamp or a fluorescent lamp capable of quickly reaching full luminous condition, an interval of at least 10 minutes is required for any conventional high-pressure metal vapor discharge lamps before it can again recover full luminous power.
To solve this problem, as is typically described in the publication of U.S. Pat. No. 4,287,454, a high-pressure sodium lamp is provided, which comprises a pair of arc tubes in an outer envelope which are electrically connected in parallel with each other. When the proposed high-pressure sodium lamp normally lights up, one arc tube in the pair of arc tubes remains lit. When power service is resumed after a momentary interruption, the other arc tube containing a low pressure lights up. In this case, while the former arc tube remains lit, the latter arc tube has a slightly raised internal pressure due to the preliminarily applied heating effect. As a result, this arc tube can start and reach its full output in a few minutes. In other words, the high-pressure sodium lamp cited above fully restarts in a very short period of time, thus offering much convenience for constantly illuminating highways and tunnels.
Furthermore, even when one of the arc tubes cannot light up itself, the other arc tube lights up. This in turn significantly extends the life of the high-pressure sodium lamp cited above. Theoretically, the service life of this high-pressure sodium lamp is twice as long as that of a conventional high-pressure sodium lamp merely housing a single arc tube.
On the other hand, when operating such a high-pressure sodium lamp incorporating a pair of parallel connected arc tubes in the outer envelop, the arc tube in the pair of arc tubes which has a lower starting voltage lights up. In other words, due to unexpected irregularities incidental to the manufacturing process, starting voltages may be slightly uneven between the two arc tubes. As a result, the arc tube with the lower starting voltage lights up first. Hence, when activating the high-pressure sodium lamp cited above, either one of the arc tubes, whichever has a lower starting voltage, always lights up first. In other words, whenever operating the high-pressure sodium lamp with a pair of arc tubes cited above, the arc tube with the lower starting voltage tends to light up first. This means that the arc tube with a lower starting voltage often light up itself, thus resulting in dissipation of the sodium filled in this arc tube. If this symptom occurs, then the lamp voltage of the arc tube rises to cause the luminous characteristic of the lamp itself to quickly degrade.
When either of these arc tube is no longer available, the other one can be operated. Since one arc tube of the pair of the arc tubes cannot instantaneously light up again, the high-pressure sodium lamp can no longer maintain the objective discussed above.
Mostly, the rise of voltage in the arc tube accounts for the generation of disabled arc tubes caused by unilateral operating. Consequently, during the latter half of the service life of the high-pressure sodium lamp, the arc tube that tends to initially light up frequently extinguishes. When this arc tube is extinct, the other arc tube starts lighting up in turn. After the former arc tube is subject to repeated extinction, leakage eventually occurs in it, thus causing the rare gas in it to leak into the outer envelope. As a result, the leaked rare gas absorbs the ignition pulse to prevent the other arc tube from being activated. For example, even when the arc tube is operated, as a result of heat dissipation by the rare gas, neither temperature nor voltage rises in the arc tube, thus lowering luminous efficiency. In the same way, the high-pressure discharge lamp incorporating a plurality of arc tubes is adversely affected by those symptoms described above.